The present invention relates to a felt conditioning system having particular application to papermaking machinery in which travelling felts absorb water from a paper or board sheet being formed by the machine. In order to assure efficient machine operation it is necessary to dewater the felt and remove other materials picked up by the felt from the paper web such as loose fibers, clays, etc.
In the press section of a papermaking machine top and bottom endless press felts are used to remove water from a paper or board sheet being formed. For proper functioning of the endless felts it is necessary to remove all water absorbed by the felt in each revolution otherwise the felt becomes supersaturated. It is particularly important to remove absorbed water from the felt before it reaches the press nip so that the felt is properly conditioned, i.e., water has been removed to enable the felt to absorb the maximum quantity of water from the paper sheet. In conventional practice it is common to see a paper machine operating with a wet nip, i.e., a back flow of water to the incoming side of the press nip--a clear indication that the felt is supersaturated. A wet nip occurs because the felt conditioning suction boxes are not removing the quantity of water taken up by the felt for each felt cycle. A supersaturated felt travelling at 3000 fpm encounters high hydraulic forces at the press nip causing removal of fines from the paper sheet and requiring reduction in nip pressure to avoid hydraulic forces which would destroy the sheet. Of course, with reduced nip pressure less water is removed from the sheet.
Accordingly, conventional techniques for conditioning felts on operating paper machines have inherent limitations so that press felts are not properly dried.
In felt conditioning with suction boxes a saturated felt passes over a vacuum opening or slot extending across the machine beneath the felt. At machine speeds of 3000 fpm any point in the felt has a dwell time of 1.6 milliseconds over a 1-inch vacuum slot. As machine speed increases the dwell time grows shorter limiting the volume of water that can be drawn by vacuum through the slot. Moreover, removal of water from a travelling felt into a suction box requires the force of air drawn through the felt to deflect each droplet of water moving with the felt at machine speed. As machine speed increases greater air force is required to remove water from the felt. To overcome these limitations and to achieve increased water removal at greater machine speeds one may use more than one suction slot, however, the cost for this improvement is reduced felt life.
In practice, suction boxes are applied to the paper sheet side of the felt because the dirt to be removed is located toward that side of the felt. The suction boxes then wear the nap of the felt and diminish the ability of the felt to absorb water. Suction boxes are also applied to a horizontal run of the top felt after the paper side of the felt has passed over an outside roll which presses the dirt into the felt before reaching the suction box.
Another technique for felt conditioning is the honeycomb roll described in U.S. Pat. No. 4,116,762 to Gardiner. According to Gardiner the felt passes over a rotating honeycomb roll while conditioning air moves through the foraminous structure of the rotating roll and through the felt. Since the honeycomb roll rotates, the conditioning air is supplied to a stationary plenum within the roll in an axial direction from both ends of the roll. Supplying air through the roll in an axial direction is not feasible because extremely high air velocities are required in order to move the necessary volume of air through the felt for conditioning. High velocity air loses pressure as it moves through the axial supply tubes with resultant loss of air temperature and volume and diminished ability for conditioning the felt. The diameter of the honeycomb roll cannot be increased to achieve greater conditioning air volume with lower air velocities because the maximum pressure of conditioning air is inversely proportional to the radius of curvature of the felt passing over the roll at a gigven felt tension. As a result any increase in honeycomb roll diameter requires lower conditioning air pressures to avoid lifting the felt away from the honeycomb roll surface.
Felt manufacturers recommend a minimum flow of conditioning air for the honeycomb roll of 6 cubic feet per minute per square inch of felt or approximately 100 cfm per inch of felt width. For a 300-inch wide felt 30,000 cfm of conditioning air is required at velocities approaching 25,000 fpm. As the conditioning air expands through a honeycomb roll under these conditions its temperature drops to the point of freezing the water carried by the felt. In addition, water viscosity increases as temperature decreases inhibiting its removal from the felt.
A further limitation of the honeycomb roll inheres in the nature of the honeycomb roll itself. As the moving felt engages the surface of the honeycomb roll, a pocket of ambient air is trapped in the cells defined by the honeycomb structure between the felt and the pressurized plenum within the roll. Felt conditioning air in the interior plenum chamber of the honeycomb roll therefore must first compress the trapped ambient air before passing through the felt. In addition, the trapped ambient air will lower the temperature of hot conditioning air. As a result of this limitation, time is lost and the effectiveness of the conditioning air is diminished. It is not likely that these air pockets can be eliminated since the honeycomb structure requires a given depth of lattice work to achieve roll strength sufficient to support the felt under tension. In addition with the current industry trend to wider machines the honeycomb structure must have greater radial dimensions to meet strength requirements. Accordingly, the honeycomb roll is limited in utility for purposes of felt conditioning by passing pressurized air through the felt and has not been commercially used in the papermaking industry.
Another felt conditioning device is disclosed in U.S. Pat. No. 3,347,740 to Goumeniouk. This device utilizes either a rotating or a stationary tube member for supplying air under pressure to fill the voids created in a travelling felt as it expels water under the influence of centrifugal force. In order to generate sufficient centrifugal force for water removal, a very small diameter tube or roll is required. Accordingly, for reasons elaborated above, felt conditioning by use of centrifugal force and by moving air through the felt are physically incompatible techniques and cannot be used together with advantage.